1. Field of the Invention
This invention pertains in general to access opening cover seals and more particularly to pressure vessels that employ access opening cover seals.
2. Description of the Related Art
Steam generators used in nuclear reactor power plants are very large heat exchangers where heat from a primary fluid heated by a nuclear reactor is transferred to a secondary fluid which is converted into steam and used to drive a turbine generator. Steam generator heat exchangers are housed inside a tall, generally cylindrical steel shell. A large number of U-shaped heat exchanger tubes are enclosed in the shell and have their ends inserted in holes formed in a horizontal tube sheet or plate near the bottom of the steel shell. The tubes are used to convey the primary fluid which has been heated in the nuclear reactor. The secondary fluid or feedwater used to generate the steam is introduced into the steam generator in such a manner that the secondary fluid flows around the outside of the heated tubes thereby converting much of the secondary fluid into steam which is allowed to exit the steam generator through an outlet nozzle at the top of the steel shell.
In the past, steam generator tubing in nuclear plants has been exposed to extreme operating conditions and were susceptible to stress corrosion cracking, mechanical wear, wall thinning and pitting. To address this susceptibility, a number of techniques have been developed to inspect steam generator tubing for degradation prior to tubing failure in order to prevent leakage of the primary radioactive coolant into the secondary side which would result in forced outages. Steam generator tubing has been most commonly inspected using a variety of eddy current methods, most involving probes which were inserted into the tubes from the underside of the tube sheet on the primary side of the steam generator. The probes are inserted through a steam generator manway in the lower hemispherical primary coolant inlet and outlet side of the steam generator below the tube sheet and into the tube sheet whereby the corresponding tubes are mapped by inserting the probes up through the tubes. In addition, to minimize the onset of stress corrosion cracking, denting and wall thinning of the tubes, maintenance is regularly performed on the secondary side of the steam generator during scheduled plant outages to remove sludge, other deposits and loose parts. These maintenance operations require access to both the primary and secondary sides of the steam generator through access openings, e.g., manways that are provided in the steam generator shell and channel head. These access openings have removable covers that necessarily have to be resealed after the maintenance operation to restore the barrier to the pressurized environment that exists in the interior of the primary and secondary side of the generator during plant operation.
The access opening covers in use currently typically employ planar insert plates or diaphragms that can respectively accommodate either gasket closures or seal welded closures. Such a design uses the same cover and threaded fasteners whether the closure was made with a gasket or seal welded diaphragm. The purpose of the insert is to provide an inside diameter restraint and sealing surface for the gasket and to provide a relief or gap between the cover and the access pad surface of the pressure vessel against which the seal is established as well as to provide a corrosion resistant barrier between the fluid and the typically ferritic cover material. If the pad gasket seating surface is damaged during operation or routine servicing during a shutdown, sealing can be provided by a seal welded diaphragm, in the event the time for a repair is not available. Seal welded diaphragms are generally only used when the gasket seating surface is damaged and repair is not practical or when access through the opening is believed to be infrequent.
When it has been necessary to use seal weld diaphragms small fillet welds are typically used to allow easy removal at the site if and when access to the steam generator internals is required. Because of the diaphragm configuration and type of weld used, qualification of the weld design to ASME B&PV Code Section III rules has not been possible. The main issue in developing a design acceptable to the Code is to achieve an acceptable Code fatigue usage for a forty or sixty year life. Since the weld is outside the Code jurisdictional boundary, meeting the Code is not mandatory. However, from a functional standpoint it is necessary to provide a design that has margin against failure of the weld which would result in a leak. Having a design that meets the ASME Code would provide sufficient margin to ensure a leak free joint.
Accordingly, it is an object of this invention to provide a seal between a pressure vessel wall and an access opening cover that satisfies ASME Code requirements.
It is a further object of this invention to provide such a seal that will reduce the stresses on the weld to lessen the likelihood of fatigue failure over the life of the seal as well as bending loads on the cover attachment hardware.
It is still a further object of this invention to provide such a seal that will last for the life of the pressure vessel in which it is installed.